We are studying the interaction between retroviral envelope and receptor proteins, which mediate viral entry. Some studies involve mutating the receptor, or the envelope, to see what portions of these proteins are involved in various steps in the fusion process. Other approaches include modifying the surface charge or lipid composition of the membranes involved in fusion. We made some interesting observations related to the latter approach over the past year. In trying to make new fluorescent probes to monitor fusion events at the cell surface, we found that attaching cholesterol (a lipid) to DNA molecules caused them to bind tightly to the surface of cells. Since DNA is negatively charged, we investigated whether cell surface-bound DNA affected virus-mediated membrane fusion. We found that cholesterol-DNA probes inhibited entry by certain strains of HIV but not others. The strains that were inhibited had a high density of positive charge in a variable region of the HIV envelope protein known as the V3 loop. Previous work had shown that dextran sulfate and heparin, other negatively charged polymers, inhibited the same strains of HIV, though at much higher concentrations, presumably because these compounds do not concentrate at the cell surface. Consistent with an electrostatic interaction, the inhibitory effects of cholesterol-DNA were blocked by a positively charged polymer, DEAE-dextran. The cholesterol-DNA probes have several unique properties that make them interesting tools to study cell surface events: they are targeted to specialized membrane domains known as rafts that are involved in cell-signaling and, probably, virus-mediated cell fusion; by adjusting the length of the DNA, they might be used to probe events at different distances from the cell surface; because they hybridize to complementary DNA strands, they could be used to engineer various types of close connection between cells with complementary probes on their surfaces.